This invention relates to image intensifier tubes and more particularly to an improved structure for supporting the high voltage substrate within an x-ray image intensifier tube.
Recently a large sized proximity type image intensifier tube has been developed and is the subject of U.S. Pat. Nos. 4,104,516 and 4,140,900. In this type of proximity image intensifier tube a scintillator-photocathode is formed on a plate which is suspended within a metallic tube envelope on insulating rods between an inwardly concave metallic input window and a phosphor output screen deposited on a glass output viewing window. A high voltage is applied to the scintillator-photocathode assembly plate through one of the insulated support rods. The high voltage potential exists between the scintillator-photocathode assembly and the metallic tube envelope. Because of the extremely high voltage applied to the scintillator-photocathode assembly plate there is the great danger of electron or corona discharge between the edge of the plate and the interior walls of the tube envelope. Such discharge is most likely at the edges of the scintillator-photocathode assembly plate which, in the device described in the above-mentioned patents, is generally disk shaped. The discharge danger is less towards the center of the plate than at its edges.
The probability of electron or corona discharge at any particular location on the plate is proportional to an exponential function of the high voltage gradient at that location. The high voltage gradient is increased at any location where the surface of the plate forms a sharp edge or point. Thus the area of attachment of the support rods potentially has a higher voltage gradient than other locations on the plate. In the tube described in the above-mentioned patents the support rods attach tangentially to the plate at locations inset from its outer, circumferential edges, i.e., on the flat surface of this plate where the high voltage gradient is less than at the plate edges.
The outside surfaces of the insulating support rods are coated with a slightly conductive material, such as chrome oxide, to bleed off the accumulated charges. The potential high voltage gradient is around or less than 20,000 volts per centimeter over the length of the support rods. Because of the potential voltages used, a certain length rod was thought necessary in order not to exceed this high voltage gradient to minimize the diameter of such high voltage image intensifier tube, however, it was thus necessary to attach rods to the high voltage place at a tangential angle, with respect to the circumference of the plate, so that the rods did not extend radially from the center line of the tube. This allowed a longer rod to be encased within the tube envelope. At the point where the support rods attached to the high voltage plate they protruded from the flat surface of the plate. This made necessary a corona shield around the edge of the plate. The use of such a corona shield and the relatively thick support rods thereby increased the length of the image intensifier tube.
The increased length of the tube increases both the weight of the tube and the height of the apparatus when it is used for fluoroscopic examination. However, a reduction in the weight of the tube allows easier and simpler construction of the fluoroscopic table and a reduction in the height, that is the length of the tube, allows easier viewing for shorter radiologists during a direct view examination.
In addition to these design problems, the prior art support structure presents a number of manufacturing problems. In the prior art substrate support design the rods were first attached to the scintillator-photocathode assembly and then the entire structure was placed within the tube. Accurate registration relative to the interior of the tube was relatively difficult and the assembling of the tube became more time consuming. Also, the welding process by which the support rods were attached was relatively time consuming and expensive. In the event that a support rod proved defective, just prior to or during the assembling of the tube, the removal and replacement of it required that the entire assembly be removed from the tube housing, thereby adding to the cost of manufacture of the tube. Still another problem was the lack of standardized parts. If the nature of the high voltage substrate was to be altered, as for example when a two stage version of the tube was developed, then different parts had to be designed to adapt to the new high voltage support structure.